CRYSTAL-STRUCTURES OF CA5CR3O12 AND CA5CR2SIO12, THE CHROMIUM ANALOGS OF SILICOCARNOTITE

The crystal structures of Ca5Cr3O12 and Ca5Cr1.8Si1.2O12, the chromium analogues of silicocarnotite, Ca5P2SiO12, have been determined. Both compounds were grown at 1250-degrees-C and analyzed by electron microprobe analysis. Diffraction data collection was done on spherically ground crystals which are both orthorhombic with space group Pnma. Charge-balance requirements as well as site-occupancy refinement of the Si-containing compound point strongly to the presence of both tetravalent and hexavalent chromium in tetrahedral sites. The Si is located together with tetravalent Cr in a general position, whereas the hexavalent Cr is situated on a mirror plane. The calcium atoms are located in seven-, eight-, and nine-coordinated sites. The presence of vacant channels of 3.5 angstrom diameter perpendicular to (100) is a feature of this structural type. Its relation to the apatite and glaserite structures is also shown.